According to the U.S. Centers for Disease Control and Prevention Zika is considered endemic in some of these areas, having existed there for years, and some residents may be immune. But it said a number of U.S. travelers have become infected in the region in the last year.

The advisory comes at the same time authorities in Thailand confirmed that two cases of babies with microcephaly, or abnormally small heads, were caused by the Zika virus, the first such cases found in Southeast Asia.

Dr. Prasert Thongcharoen, a senior Health Ministry official, said in a statement Friday that the linkage to Zika was confirmed by laboratory tests in two of three cases of babies afflicted with microcephaly. The results were inconclusive in the third case.

The World Health Organization urged countries in the region to take stronger measures to contain the virus. The U.N. agency said the cases are the first of Zika-associated microcephaly in Southeast Asia.

“Zika virus infection is a serious threat to the health and wellbeing of a pregnant woman and her unborn child. Countries across the region must continue to strengthen measures aimed at preventing, detecting and responding to Zika virus transmission,” WHO Southeast Asia regional director Dr. Poonam Khetrapal Singh said in a statement.

Zika generally causes a mild flu-like illness, but a major outbreak in Brazil last year revealed that it can result in severe birth defects when pregnant women are infected. There is no vaccine or specific treatment for the virus.

The disease is spread primarily by mosquitoes, and WHO urged private citizens as well as governments to take strict mosquito control measures.

Mosquitoes are a constant concern in Thailand because they also transmit malaria, dengue fever and chikungunya.

Statistics issued by Thai health officials show more than 300 confirmed Zika cases since the start of the year.

The CDC says that the level of risk for Zika virus infection in the 11 countries in the travel advisory is unknown, but “it is likely lower (but not zero) than in areas where Zika is newly introduced and spreading widely.” However, because Zika can cause severe birth defects, pregnant women should talk to their healthcare provider and consider postponing nonessential travel to Southeast Asia.

WHO said travelers to areas with Zika virus outbreaks “should seek up-to-date advice on potential risks and appropriate measures to reduce the possibility of exposure to mosquito bites​ and sexual transmission of Zika​.” But it advised pregnant women “not to travel to areas of ongoing Zika virus transmission.”

“Pregnant women’s sexual partners living in or returning from areas with Zika virus outbreaks should ensure safer sex or abstain from sex for the duration of their partner’s pregnancy,” it said.